RDMA/rw: Fix MR pool exhaustion in bvec RDMA READ path

When IOVA-based DMA mapping is unavailable (e.g., IOMMU
passthrough mode), rdma_rw_ctx_init_bvec() falls back to
checking rdma_rw_io_needs_mr() with the raw bvec count.
Unlike the scatterlist path in rdma_rw_ctx_init(), which
passes a post-DMA-mapping entry count that reflects
coalescing of physically contiguous pages, the bvec path
passes the pre-mapping page count. This overstates the
number of DMA entries, causing every multi-bvec RDMA READ
to consume an MR from the QP's pool.

Under NFS WRITE workloads the server performs RDMA READs
to pull data from the client. With the inflated MR demand,
the pool is rapidly exhausted, ib_mr_pool_get() returns
NULL, and rdma_rw_init_one_mr() returns -EAGAIN. svcrdma
treats this as a DMA mapping failure, closes the connection,
and the client reconnects -- producing a cycle of 71% RPC
retransmissions and ~100 reconnections per test run. RDMA
WRITEs (NFS READ direction) are unaffected because
DMA_TO_DEVICE never triggers the max_sgl_rd check.

Remove the rdma_rw_io_needs_mr() gate from the bvec path
entirely, so that bvec RDMA operations always use the
map_wrs path (direct WR posting without MR allocation).
The bvec caller has no post-DMA-coalescing segment count
available -- xdr_buf and svc_rqst hold pages as individual
pointers, and physical contiguity is discovered only during
DMA mapping -- so the raw page count cannot serve as a
reliable input to rdma_rw_io_needs_mr(). iWARP devices,
which require MRs unconditionally, are handled by an
earlier check in rdma_rw_ctx_init_bvec() and are unaffected.

Fixes: bea28ac14cab ("RDMA/core: add MR support for bvec-based RDMA operations")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260313194201.5818-3-cel@kernel.org
Signed-off-by: Leon Romanovsky <leon@kernel.org>